1. Field of the Invention
An amine-cured silicone composition resistant to exposure to hydrocarbon fluids and other harsh environments is provided herein and, in particular, an amine-cured silicone composition containing substantially no plasticizer and which is resistant to a transmission fluid at high temperatures.
2. Description of the Prior Art
Room-temperature vulcanization (“RTV”) silicone compositions can be used as sealants and adhesives in the automotive industry. A common use of RTV silicone sealants is with electronic devices that are submerged in a synthetic hydrocarbon fluid, such as a transmission fluid, where these devices may require some type of seal about their housing in order to prevent the transmission fluid from ingressing into the electronic cavity. The sealing area can include the housing, connector, metal leads, and the like.
The electronic devices may be used in automotive transmission assemblies to control the operation of the transmission system. The electronic devices can be placed in housings that are typically partially or completely submerged in the transmission fluid within the assembly. One or more electric leads typically enter and exit the housing to supply power or carry control signals to or from the electronic devices within the housing or to and from other components outside the housing. If the electronic device housings are not properly sealed, the transmission fluid can leak into the housing. If the transmission fluid reaches the electronic devices, the devices may become disabled or may malfunction and thereby disable the system, which can alter the performance of the vehicle.
One method of preventing the transmission fluid from leaking into the electronic device at around the entry points of the electric leads is to utilize a silicone sealant associated with the leads. Typically, a sealing cell or cavity can be formed about the leads where the leads enter the housing. After the sealing cell or cavity is formed, an adhesive or sealant can be applied into the cell or cavity in an attempt to prevent fluid entry into the electronic device. However, the sealants commonly used can debond from the sealing cell and/or become degraded.
Commonly used sealing materials in transmission systems can include Platinum (“Pt”)-cured silicones, conventional amine-cured RTV silicones, acrylics, urethanes, and fluoro-silicones; unfortunately, these sealing materials are not effective in most commonly used transmission fluids, such as Dexron® VI (used by General Motors), due to degradation and/or high-temperature instability. When typical RTV sealant compositions are exposed to hot transmission fluid, including, for example, the synthetic transmission fluid Dexron® VI, the composition itself often becomes degraded and its bonding strength to the surface decreases. Additionally, epoxy-based sealants can have improved stability in certain transmission fluids, however, they frequently become brittle with prolonged exposure and have a lower ionic purity that can lead to resistance drops at elevated temperatures. Perfluoropolyether based adhesives, such as Sifel®, manufactured by Shin-Etsu Chemical Co., Ltd., may have sufficient stability when immersed in certain transmission fluids, but do not provide the necessary initial adhesion to plastics or aluminum used to house the electronic devices. Furthermore, the cost of perfluoropolyethers is approximately tenfold as expensive compared to common silicone sealants not to mention its limited world supply.
Moreover, platinum-cured silicones are found to be relatively unstable upon exposure to transmission fluid while a conventional amine-cured silicone can have greater stability in comparison. It is believed that the reason for this instability of Pt-cured silicones is due to the carbon-carbon segment in the backbone structure; whereas the RTV silicone backbone structure comprises silicon-oxygen (Si—O) bonds. The structural similarity of the carbon-carbon bond to hydrocarbon-based transmission fluid makes Pt-cured silicone easier to dissolve and swell in the synthetic transmission fluid and makes it more prone to the attack of either base, acid, or free radical species.
Commercially available RTV silicone sealants do not currently exhibit desirable results in all areas of adhesion and resistance to various surfaces when exposed to transmission fluid. In general, oxime-cured and alkoxy-cured sealants debond from the substrate, whether aluminum or plastic. The conventional amine-cured sealants can exhibit bonding to either substrate, yet exhibit cohesive failure with peeling. Additionally, the conventional amine-cured sealants exhibit the greatest weight loss when compared to the other sealants.
Automatic transmission fluid (ATF) is considered the life-blood of the automatic transmission, therefore replacing the transmission fluid with something less harsh on the sealant material is not feasible. ATF is used to transfer power in the torque converter and provide the pressure to apply the transmission's various clutches and bands. It also is used to clean, lubricate and cool the components of the transmission. Another use of the RTV silicone sealants can be as gasket sealants and/or as formed-in-place gaskets for use in internal combustion engines.
Although conventional amine-cured RTV silicone sealants exhibit some amount of stability after exposure to transmission fluid, the conventional amine-cured RTV has a significant amount of weight loss upon prolonged exposure to the transmission fluid, as well as contributing to foaming within the transmission assembly. Most transmission fluids already contain an anti-foaming agent therein, however, when the conventional amine-cured silicone sealant is introduced, the anti-foaming agent becomes ineffective and foaming results. Typically, foam is harder to cool down than a liquid and therefore the transmission fluid cannot be cooled properly and a thermal runaway condition in the transmission is possible, e.g., the transmission assembly can overheat. As a result, automotive manufacturers have recommended against using silicone sealants in these environments with the belief that these silicone compounds are responsible for these conditions and hence are ineffective and can be “attacked” in these environments. However, the true cause for the foaming issue is not thoroughly understood in the art. Furthermore, the conventional amine-cured RTV silicone sealants contain a substantial amount of a plasticizer.